1. Field of the Invention
The present invention relates to an image printing apparatus and an image printing method for forming an image on a printing medium with a plurality of dots of different sizes.
2. Description of the Related Art
Ink jet printing apparatuses are widely in use as information outputting device such as printers, copy machines and facsimiles. An ink jet printing apparatus includes a print head for ejecting ink as a droplet, and forms an image on a printing medium such as a paper sheet or a thin plastic plate, by printing dot patterns based on image information.
In a case of an ink jet printing apparatus, since an image is represented by printing or not printing a dot, granularity of dots in a highlight area (termed a lowest tone area) has been considered as a problem. To solve this problem, there is proposed an apparatus of printing images with a plurality of inks that are different from each other in terms of a density of coloring materials, like inks of cyan and light cyan, and inks of magenta and light magenta (for example, see Japanese Patent Laid-Open No. 2003-300312). With such an apparatus, granularity can be reduced by using inks of light cyan and light magenta in a highlight area. An increase of kinds of used inks (consumable items), however, leads to an upsizing of an apparatus and an increase of running costs.
On the other hand, a demand for printing an image in much higher resolution has been increasing. To satisfy this demand, a printing element in a print head has been improved with higher definition and smaller droplets. In recent years, there have been provided a large number of ink jet printing apparatuses each capable of ejecting ink droplets of 1 to 2 pl with high density and high resolution of more than 1200 dpi. A printing apparatus capable of making a print with higher definition and smaller droplets can achieve both a reduction in the foregoing granularity and an increase in resolution of images, concurrently.
When the higher resolution of an image is achieved, however, the number of pixels that should be subjected to image processing increases, and this may result in an increase of a load and a processing time of the image processing. Nevertheless, this problem can be solved by introducing binarization processing called “INDEX patterning processing” to the image processing.
In general, image data to be printed by a printing apparatus is represented with luminance data containing multiple values such as R (red), G (green) and B (blue). In contrast, in a case of an ink jet printing apparatus, an image is represented with dots printed or not-printed by using inks such as C (cyan), M (magenta), Y (yellow) and K (black). For this reason, various steps of image processing are needed for converting the multivalued luminance data (RGB) into binary density data (CMYK). The various steps include processing of converting multivalued luminance data (256 values, for example) into multivalued density data (similarly, 256 values), processing of converting the multivalued density data into density data using a smaller number of levels (5 values), and the like. Moreover, the various steps also include the INDEX patterning processing of converting the density data using a smaller number of levels (5 values) into binary density data.
FIG. 1 is a schematic diagram for explaining the INDEX patterning processing. FIG. 1 shows patterns for converting density data of 5 values (levels 0 to 4) with a resolution of 600 dpi into binary (print/not-print) density data with a resolution of 1200 dpi. In this example, 1 pixel of 600 dpi is the minimum unit for the image processing before the INDEX patterning processing, and 1 pixel of 1200 dpi is the minimum unit for specifying whether or not to print a dot after the INDEX patterning processing. 1 pixel of 600 dpi is equivalent to an area of 2 pixels×2 pixels of 1200 dpi. The higher the level (the density value), the greater the number of dots printed. By providing the INDEX patterning processing like this to the final stage of the image processing including various steps, the number of pixels that should be treated in the image processing therebefore can be reduced. This results in a decrease in a load and a processing time of the entire image processing. Hereinafter, in this description, a resolution (600 dpi in this example) before the INDEX patterning processing is referred to as an image resolution, and a resolution (1200 dpi in this example) after the INDEX patterning processing is referred to a printing resolution. In other words, 1 pixel of the image resolution is an area that can be represented with n (n is an integer at least 3) levels of density (n tones), and 1 pixel of the printing resolution is an area that can be represented with 2 levels of density (dot-on/dot-off). Accordingly, the employing of the aforementioned INDEX patterning processing allows an image to be outputted with small droplets, a high printing resolution and less granularity.
Nevertheless, there is a problem specific to a print head that can eject only small droplets. Indeed, a single small dot formed on a printing medium by a small droplet of ink is not likely to be noticed, and thus reduces the granularity in a highlight area. However, a larger number of dots need to be printed in order to represent a sufficiently high density. This results in an increase of the number of times that the print head ejects droplets, and thereby leads to conditions that are more likely to increase the temperature of the print head. It is desirable that the temperature of the print head be kept within a predetermined range in order for the print head to eject ink droplets normally and stably. When the temperature of the print head increases too much, an action of suspending the printing operation or the like is taken in general. Such a suspension, however, leads to a decrease of a printing speed.
In order to solve the foregoing problems, a printing method using a print head capable of ejecting ink droplets of several different size levels has been proposed in recent years, and a printing apparatus employing such a method has also been provided. A printing apparatus capable of ejecting ink droplets of several different size levels can efficiently carry out gradation representation while curbing granularity, by printing small dots in a highlight area, and by printing large dots in a high density area. In addition, this method does not require the upsizing of an apparatus, and an increase of running costs unlike a case of additionally using light color inks each having a low density of coloring materials.
An effective structure for a print head capable of printing with high density at a high speed is one provided with a heater (an electrothermal element) in an ink path in each printing element. In such a print head, a bubble is generated in an ink by applying a voltage pulse to a heater, and then the ink is ejected from an ejection port with growing energy of the bubble.
Japanese Patent Laid-Open No. Hei 10-071730 discloses an ink jet printing apparatus which includes a heater for a large dot and a heater for a small dot in each printing element, and which thereby is capable of printing both large and small dots in the same print scan. In addition, Japanese Patent Laid-Open No. 2004-148723 discloses the scheme in which image data for large dots and image data for small dots are each independently quantized to reduce the number of levels and then are respectively assigned dot matrix patters (INDEX patterns) that are independently prepared. This patent document describes the scheme in which the dot matrix patterns are determined so that large and small dots are not printed overlappingly in the same pixel of a printing resolution.
Moreover, Japanese Patent Laid-Open No. 2004-160913 discloses an apparatus for printing an image with a print head capable of printing dots of three size levels, that is, small, middle and large. This patent document describes a scheme for making an adjustment of each apparatus or making an adjustment depending on an age deterioration of a print head, for the purpose of curbing banding in the following manner. Firstly, a plurality of patterns with different mixing ratios of small, middle and large dots are printed. Then, among the printed pattern images, one having less banding is selected and set for printing.
In this way, the use of a print head capable of ejecting ink droplets of several different size levels, and the introduction of the INDEX patterning processing results in achievements of reduction of granularity in a highlight area, effective tone representation in a high density area and speedup of image processing and printing operation, all together.
However, even in a case of employing the aforementioned techniques, it has been recently considered as a problem that banding is easily noticed in a tone area in which the density is mainly represented by a large number of small dots. This phenomenon has harmful effects especially on serial-type ink jet printing apparatuses used in a wide range of fields.
In a serial-type printing apparatus, an image is formed intermittently by alternately performing main scans and subsub scans. Here, in the mainmain scan, a print head moves relative to a printing medium while ejecting ink. On the other hand, in the subsub scan, the printing medium is conveyed by a predetermined amount in a direction orthogonal to a direction of the mainmain scan. With this configuration, the serial-type printing apparatus has various advantages that: the serial-type apparatus can be downsized more than other types; various sizes of printing media can be handled; colors can be relatively easily increased; a speed and printing quality can be adjusted easily by introducing a multi-pass print mode; and the like.
However, the conveying amount in a sub scan inevitably varies to some extent due to the eccentricity of rollers conveying a printing medium and the like. Under this condition, when smaller dots are uniformly printed, variations in the conveying amount generate dot-dense portions and dot-sparse portions in the sub scan direction. These dot-dense and dot-sparse portions are more likely to be noticed as density unevenness.
Hereinafter, the aforementioned harmful effect will be described more specifically by referring to the drawings.
FIG. 2 shows a schematic diagram for explaining INDEX patters used in an ink jet printing apparatus that forms an image with large and small dots. FIG. 2 shows patterns each specifying whether or not to print large and small dots in each printing pixel in a printing resolution of 600 dpi (vertical)×1200 dpi (horizontal), corresponding to density data having 7-valued levels with an image resolution of 600 dpi. The width of 1 pixel of 600 dpi is approximately 42 μm, and that of 1200 dpi is approximately 21 μm. On the other hand, the diameter of a large dot used in this example is 60 μm, and that of a small dot is approximately 35 μm.
The left side of the table shown in FIG. 2 shows the numbers of small dots and large dots to be printed in 1 pixel of the image resolution, corresponding to each level. The right side of the table shows a dot printing state corresponding to each of the levels. It is obvious that dots to be printed increase in number and size as the level becomes higher. Here, pay attention to the level 2. The level 2 is a tone value that is formed only by small dots in this example.
FIGS. 3A and 3B are diagrams showing dot alignment states in a case where data of the level 2 is continuously printed on a certain range of area. To print this, a multi-pass printing method is employed, and multiple small dots in the area are printed in multiple main scans with multiple sub scans each performed between the main scans. FIG. 3A shows a state printed without variation in the multiple sub scans, and FIG. 3B shows a state printed with variations therein.
The diameter of the small dot (35 μm) is smaller than the width (42 μm) of 1 pixel of the image resolution. Accordingly, if there is no variation in the sub scans, as shown in FIG. 3, the small dots aligned in the sub scan direction are not in contact with each other, and lines are formed extending in a main scan direction with white background portions sandwiched from above and below. In addition, the presence of the white background portions means that coverage (a ratio of an area covered with ink to an entire area for printing an image) on a printing medium is less than 100%. The presence of the white background portions increases the lightness.
On the other hand, if there are variations in the sub scans, as shown in FIG. 3B, the small dots aligned in the sub scan direction are arranged in contact with or away from each other, and small white background portions are formed irregularly. In this case, coverage on a printing medium is greater than in the case shown in FIG. 3A, and thereby the lightness is lowered. Such lightness and coverage are influenced by a contact between ink droplets before being fixed on a printing medium.
FIGS. 4A and 4B are magnified diagrams focusing on boundary areas in the sub direction shown in FIGS. 3A and 3B, respectively. As shown in FIG. 4A, when a white background portion exists between dots aligned in the sub scan direction, the dots aligned in the sub scan directions are not in contact with each other, and maintain a distance therebetween. On the other hand, in the case where there are variations in the sub scans, portions where dots aligned in the sub scan direction are in contact with each other appear as shown in FIG. 4B.
If the ink droplets are brought into contact with each other before being absorbed by the printing medium, the ink droplets are attracted to each other by their surface tension, which causes a phenomenon in which the ink flows from one of the ink droplets into the other thereof. In other words, the ink flows in main scan directions but does not flow in the sub scan direction in the case of FIG. 4A, while the ink flows both in the main and sub scan directions in the case of FIG. 4B. When such a flowing phenomenon occurs, the dots become deformed, and change the shape so as to expand the covering area. As a result, the coverage is increased. In other words, a contact between ink droplets is a factor of further increasing a change of the coverage due to variations in sub scans.
If there are variations in sub scans, the coverage and lightness also vary by conveying width on a printing medium. When a conveyance roller is decentered, the variations in the conveying amount periodically appear, and thereby the variations in lightness also appear periodically in the sub scan direction. Since a human visual sense is sensitive to the variations in lightness, such a phenomenon is noticed as banding or density unevenness, and is an important problem in regard to image quality.
Another method has been proposed for printing small dots shifted in the sub scan direction in order to eliminate a white background portion continuously extending in a main scan direction, at a tone level at which a print is made only with small dots. However, for the purpose of printing dots while aggressively shifting the dots in the sub scan direction, this method requires that printing elements be arranged with higher density in a print head, or that the conveying amount in the sub scan performed between two successive main scans be set so that dots can be arranged in shifted positions. The former case leads to an increase of costs for a print head since a larger number of printing elements need to be arranged with high density. In the meantime, the latter case results in an increase of costs for a printing apparatus, itself, since the printing apparatus is consequently required to convey printing media with higher definition.
As described above, Japanese Patent Laid-Open No. 2004-160913 discloses a technique of reducing banding in a way that a plurality of patterns with different mixing ratios of dots of different sizes are printed firstly, that then one of the printed images having the least banding is selected, and that the pattern of the selected image is set to actually make a print. However, in this case, an adjustment step for reducing banding is needed in addition to a normal printing operation, which may make it less easy for users to make a print. In addition, as compared with general printing apparatuses, a printing apparatus employing this technique additionally requires a large number of device, such as device for printing a plurality of patterns with different mixing ratios of dots of different sizes, and device for modifying image processing according to obtained values for adjustment. The providing of a large number of device results in an increase in complicatedness of control in an apparatus main body and a host computer, and an increase of costs for a printing apparatus.
Any of the methods described above accompanies an increase in complicatedness of control and a large increase in costs, and accordingly is not practical. For this reason, there is a demand for a simpler and more secure method for reducing banding with low duty.